This invention relates to a high thermal expansion alloy and, more particularly, to an austenitic nickel-chromium-manganese ferrous-base alloy. The alloy is especially well suited for use as the high expansion element of a temperature-sensitive bimetal strip that can be used, for example, to indicate temperature changes of the bimetal or to indicate changes in electrical current through the bimetal.
A high thermal expansion alloy, which has long been used in temperature-sensitive bimetals, has been sold under the designation 22-3 alloy. This alloy has a nominal composition of about 0.1 weight percent (w/o) carbon, 0.5 w/o manganese, 0.25 w/o silicon, 3.1 w/o chromium, 22 w/o nickel and the balance iron except for incidental impurities. 22-3 alloy features a high coefficient of thermal expansion and good atmospheric corrosion resistance. Essential to these properties of 22-3 alloy has been its austenitic microstructure at room temperature.
However, in making a temperature-sensitive bimetal from 22-3 alloy, the alloy has typically been subjected to the following processing: (a) mechanically bonding (e.g., cold bonding) a strip of the alloy to a strip of a low thermal expansion alloy such as the 36% nickel, balance iron alloy sold under the trade designation Invar "36"; (b) sintering the resulting mechanically bonded bimetal laminate in a reducing atmosphere (e.g., dissociated ammonia or hydrogen) to enhance, by diffusion, the mechanically produced bond; and (c) then cold rolling the laminate significantly to a reduced thickness which is generally more than about 50% thinner, and frequently more than about 70% thinner, than before cold rolling. During such processing, the microstructure of 22-3 alloy frequently has been partially transformed from austenite to a ferritic crystal structure such as martensite. This has reduced the thermal expansivity of 22-3 alloy and thereby reduced the flexivity of the bimetal made from the alloy. In this regard, there has frequently been a significant transformation of austenite to a ferritic crystal structure in 22-3 alloy and a resultant significant reduction in the expansivity of the alloy: (a) when the alloy has been significantly decarburized during the sintering in the reducing atmosphere and subsequently cooled to room temperature; and/or (b) when the alloy has been significantly cold worked by being cold rolled to more than about a 50% reduction in thickness.
Hence, a high expansion, austenitic alloy has been sought which can be used in a temperature-sensitive bimetal as a substitute for 22-3 alloy. In this regard, an alloy has been sought with: (a) properties, such as thermal expansivity, electrical resistivity, corrosion resistance, tensile strength and hardness, that are comparable to 22-3 alloy; but (b) more resistance than 22-3 alloy to the transformation of its austenite to a ferritic crystal structure when the alloy is significantly decarburized and/or cold worked.